Die and method for drawing metal tubes

ABSTRACT

A TUBE-DRAWING DIE AND METHOD OF DRAWING TAPERED TUBING USING A CONVENTIONAL DRAW BENCH AND A DIE ASSEMBLY HAVING A DIE RING IN THE FORM OF A PERIPHERALLY GROOVED NYLON SPOOL HELD IN A CASE DEFINING A PRESSURE CHAMBER AROUND THE SPOOL. A TUBE TO BE TAPERED IS TELESCOPED ONTO A TAPERED MANDREL AND INSERTED IN THE DIE, AND THE CHAMBER IS FILLED WITH FLUID UNDER SUBSTANTIALLY CONSTANT PRESSURE THAT IS MAINTAINED BY A RELIEF VALVE OR AN ACCUMULATOR, WHILE THE TUBE IS DRAWN THROUGH THE DIE, THE PRESSURE BEING SUFFICIENT TO COLLAPSE THE TUBING INITIALLY AROUND THE MANDREL AND THEN TO IRON THE TUBING SMOOTHLY ONTO THE MANDREL DURING DRAWING. THE ENDS OF THE DIE ORIFICE FLARE ARCUATELY AWAY FROM THE CENTRAL LAND, WHICH IS THE AREA OF MINIMUM DIAMETER OF THE ORIFICE AND IS FORMED ON A YIELDABLE ANNULUS HAVING A WALL THICKNESS INCREASING PROGRESSIVELY IN EACH DIRECTION FROM THE LAND AREA. TUBING IS TAPERED FROM EITHER END OF THE MANDREL, USING A REVERSELY TAPERED MANDREL EXTENSION WHEN THE DIE IS TO BE GUIDED ONTO THE LARGER END, AND A MODIFIED FORM OF THE CASE HAS AXIALLY RIGID, RADIALLY YIELDABLY BRACING FINGERS IN THE GROOVE AROUND THE SPOOL.

March 16, 1971 R, A., MATTHEWS 3,570,297

yDIE AND METHOD FOR DRAWING METAL TUBES Filed Sept. 19, 1968 2 Sheets-Sheet 1 rraeA/Es Mardi 16, 1971 R, A, MATTHEWS 3,570,297

DIE ANUMETHOD FoR DRAWING METAL TUBES Filed Sept. 19, 1968 2 Sheets-Sheet 2 rraewf V5 United States Patent O 3,570,297 DIE AND METHOD FOR DRAWING METAL TUBES Raymond A. Matthews, 2332 Via Anacapa, Palos Verdes Estates, Calif. 90274 Filed Sept. 19, 1968, Ser. No. 760,823 Int. Cl. B21c 3/ 06 U.S. Cl. 72-276 17 Claims ABSTRACT OF THE DISCLOSURE A tube-drawing die and method of drawing tapered tubing using a conventional draw bench and a die assembly having a die ring in the form of a peripherally grooved nylon spool held in a case defining a pressure chamber around the spool. A tube t be tapered is telescoped onto a tapered mandrel and inserted in the die, and the chamber is filled with fluid under substantially constant pressure that is maintained by a relief valve or an accumulator, while the tube is drawn through the die, the pressure being sufiicient to collapse the tubing initially around the mandrel and then to iron the tubing smoothly onto the mandrel during drawing. The ends of the die orice flare arcuately away from the central land, which is the area of minimum diameter of the orifice and is formed on a yieldable annulus having a wall thickness increasing progressively in each direction from the land area. Tubing is tapered from either end of the mandrel, using a reversely tapered mandrel extension when the die is to be guided onto the larger end, and a modified form of the case has axially rigid, radially yieldably bracing fingers in the groove around the spool.

BACKGROUND OF THE INVENTION This invention relates generally to the drawing of sections of metal tubing through die assemblies to change the cross-section of the tubing, and has particular reference to a method and die assembly primarily intended for the drawing of metallic tube sections from a substantially uniform starting diameter to a tapered finished form.

In the past, the tapering of metal tubing for Various articles, ranging from relatively small items such as golf club shafts to larger items such as lamp posts, has been a slow, expensive and often complicated operation requiring special machinery and sometimes involving compromises such as the progressive stepping of the tube diameter, the welding of a longitudinal seam to form sheet material into tapered tubes, or the use of so-called sacrificial dies which are usable usually for only one tapering operation, or at most a few, before being discarded or reshaped for further use. Spinning or rolling of tapered tubes is another approach that has been satisfactory for some purposes, but this type of operation requires extremely expensive machinery and produces a tube that may have circumferential imperfections or faults likely to fracture under bending stresses.

An early approach to the tapering of metal tubing used expansible, soft metal die rings which were capable of stretching as a tube and tapered mandrel were forced through the ring, so that the ring expanded progressively while collapsing the tube around the mandrel to a taper conforming to the taper of the mandrel. An example of this approach is Pat. No. 252,423. A more recent adaptation of this approach is shown in Pat. No. 3,327,513 in which an expansible drawing ring, composed of special metal, is used to collapse tubing onto a tapered mandrel. The die ring in this patent is said to be usable for tapering more than one tube before work-hardening of the special metal renders it incapable of proper expansion and contraction.

SUMMARY OF THE INVENTION The primary object of the present invention is to provide an improved method and die assembly for drawing ductile metal tubing onto tapered mandrels for a smooth, continuous taper and constant wall thickness, using a variable orifice die that is capable of tapering a large number of workpieces and is highly versatile with respect to the sizes, tapers and materials that can be handled. Another object is to provide an improved method and die assembly that are readily adaptable and repeatedly usable for tapering both from small-to-large and from large-tosrnall, and having much greater production capabilities than prior expansible dies. Still another object is to eliminate the need for complicated machinery and special lubricants required by conventional tapering processes, and also to avoid circumferential distribution of defects in the tubing. Thus, `the objective of the invention may be summarized as the provision of a commercially practical die and method for mass-production tapering of tubing of various types and sizes in which a single die is usable for a prolonged period without rupturing, hardening or other adverse affect on the die.

The present invention is based upon the discovery that specially shaped rings of non-metallic plastic materials, such as hard and tough nylon which has long been known and available to the trade, are hard enough, when properly formed and backed with substantially uniform pressure, to collapse ductile metal tubing of various cornpositions and wall thicknesses around tapered mandrels, and to draw a tube smoothly to the taper of the mandrel with very low friction between the die ring and the tubing. Moreover, such rings are capable of repeated plastic expansion and contraction within a substantial range of movement without rupturing or work-hardening. In its more detailed aspects, the invention also resides in the special configuration of the die ring, the backing of the ring with fluid pressure that is maintained substantially constant in a very simple maner as the orifice of the die changes with the diameter of the tubing, and in the bracing of the die for special heavy-duty drawing operations.

Other objects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a fragmentary side elevational view of a tubedrawing apparatus including a die assembly embodying the novel features of the present invention and illustrating the first step of the method, parts of the apparatus being broken away and some parts being shown in cross-section for clarity of illustration.

FIG. 2 is a fragmentary side elevation generally similar to part of FIG. l showing the entire length of the tube to be drawn, and partly broken away and shown in crosssection.

FIG. 3 is an enlarged fragmentary cross-sectional view of the die assembly as shown in FIGS. l and 2.

FIG. 4 is an enlarged fragmentary cross-sectional view of parts shown in FIG. 2 with the parts moved to the position indicated by the line 4-4 of FIG. 2, and illustrating a condition of the die ring that is believed to occur in use.

FIG. 5 is a view similar to FIG. 4 with the parts moved to the position indicated by the line 5-5 of FIG. 2.

FIG. 6 is an enlarged break-away side elevation of a representative mandrel.

FIG. 7 is an enlarged break-away side elevation of a representative Section of tubing to be tapered, the tube section being shown partly in cross-section.

FIG. 8 is a break-away side elevation of the tube section after tapering and finishing operations.

FIG. 9 is a view similar to FIG. 2, and primarily in cross-section, showing the die assembly in condition for beginning of the tapering of a tube section from the larger end toward the smaller end.

FIG. 10 is a view similar to FIG. 9 illustrating an intermediate position and condition of the die assembly in the tapering of the tube section.

FIG. 11 is a View similar to part of FIG. 10 but showing the condition of the die assembly near the end of the tapering operation.

FIG. 12 is a cross-sectional view taken in a perpendicular transverse plane through a modied form of the die assembly having special bracing elements.

FIG. 13 is a cross-section taken substantially along the line 13-13 of FIG. 12.

FIG. 14 is an enlarged perspective view of one of the bracing elements.

As shown in the drawings for purposes of illustration, the invention is embodied in a die assembly 10 mounted on a post 11 upstanding from a conventional horizontal draw bench 12 with a section 13 of tubing telescoped onto a tapered mandrel 14 ready to be drawn. In the usual fashion, one end portion 15 of the tube section is pointed to a reduced diameter so as to extend loosely through a die ring 17 in the die assembly and into a horizontal bore 18 in the post, where it is gripped in a chuck 19' on the end of a rod 20 connected at 21 to the reciprocating piston rod 22 of a horizontal hydraulic cylinder 23.

The cylinder 23 is pivotally anchored at 24 on the lefthand portion of the bench 12 so that admission of uid under pressure to the right end portion of the cylinder through a supply line 25 shifts the piston rod 22 to the left to draw the chuck 19 to the left and pull the tube 13 through the die ring 17. A second fluid line 27 is connected to the left end of the cylinder to effect the return stroke of the piston rod. It should be understood that this draw-bench arrangement is merely representative of various types of conventional benches that are well known in the art, and that the construction of the bench is not part of the present invention.

During the forward stroke of the piston rod 22, to the left as viewed in the drawings, the tube 13 and mandrel 14 are pulled together through the die assembly 10, which herein is held stationary in a recess 28 in the right side of the post 11. Thus, the tube is sized and shaped by the orice of the die ring 17 around the mandrel inside the tube.

In accordance with the primary aspect of present invention, the die ring 17 is composed of relatively hard and tough, non-metallic plastic material capable of expanding and contracting repeatedly in annular form within a substantial range without rupturing or hardening, and has a central through-passage (see FIG. 3) including a land 29 between the ends of the passage and a bell 30' flaring from the land toward the entry end. Moreover, a substantially constant compressive force is uniformly distributed around a radially yieldable annulus 31 of plastic material having the land on its inside surface, and the die ring as a whole is rigidly supported within the die assembly 10 against axial yielding under the drawing force exerted by the cylinder 23.

With this arrangement, it has been found that relatively hard plastics have the capability of crushing ductile metal tubing against a mandrel when backing pressure commensurate with the tubing strength s applied to the outside of the yieldable annulus 31, and also can withstand the axial forces tending to extrude the land area out of the die as the tubing is drawn through. Thus, the crushing compressive force is applied along the full length of the tube to taper the latter on the mandrel, without any destructive effect on the ring. In addition, it is believed that the special configuration of the bell 30 and the varying thickness of the annulus adjacent the land result in a novel buckling or rolling action of the annulus to cause the effective land area to move axially Awithin the die ring while maintaining approximately the same land width,

4 thereby preventing an increase in frictional resistance to movement of the tube as the tube diameter increases. In any event, the die assembly and method of the present invention have the demonstrated ability to taper ductile tubing sections of different thicknesses and compositions effectively over prolonged periods of production use.

In this instance, the die ring 17 is in the form of a nylon spool having axially spaced, annular flanges or heads 32 (see FIGS. 3-5) at the ends of the spool, separated by a peripheral groove 33 formed with a curved bottom surface 34 (FIG. 3), the groove preferably being generally centered axially of the spool. The through-passage constituting the variable orifice of the die extends axially through the spool with the bell 30 in the right end portion of the passage and a relief 3S in the left end portion, the bell and the relief herein being virtually identical and flaring arcuately in both directions away from the central land 29.

While it is not necessarily intended to limit the invention to such arcuate aring, it is believed that the curvature of the bell, coupled with the curvature of the bottom surface 34 of the groove 33, enhances the operation of the die ring 17 by producing the progressive rolling action of the land, as previously mentioned and illustrated in FIGS. 4 and 5. Although the land 29 initially is a circular line where the bell 30 and relief 35 merge with a smooth curvature, as indicated in FIG. 3, and may atten somewhat against the tube when backing pressure suicient to collapse the tube 13 around the mandrel 14 is exerted in the groove 33, the progressive increase in the diameters of the mandrel and the tube in the die, during drawing, results in engagement of the tube with the die ring a progressively increasing distance to the right from the original line. Instead of merely flattening out between the original line and the moving point of engagement, however, the central portion of ring appears to buckle out- Nvardly away from the tube, as permitted by the space in the groove 33, to maintain a relatively narrow land in engagement With the tube, and thereby maintaina substantially constant working force on the tube.

To achieve this result, the yieldable annulus 31 has been designed with the narrowest section backing the original land and the thickness increasing gradually to the right from this section toward the entry end of the passage. It will be seen that this is achieved by centering the curvature of the bottom surface 34 of the groove 33 on the arcuate inner surface of the bell 30 and the yieldable annulus, both centers lying generally in a plane perpendicular to the axis of the ring and the groove being narrower than the annulus. The result is the rolling, travelling action of the land which remains relatively narrow ywhile the central portion of the passage Wall lifts itself out of rubbing engagement with the portion of the tube thait already has been tapered, as shown in FIGS. 4 an 5.

The preferred means for applying uniform backing force to the exible annulus 31 is fluid pressure. For this purpose, and also to provide rigid axial support for the ring 17 as a whole, the die assembly 10` includes a case encircling the ring and cooperating twith the groove 33 to define a sealed chamber encircling the annulus to confine fluid under pressure, from a suitable source such as a pump (not shown), around the annulus.

Herein, the case comprises a cylindrical body 37 of the same length as the spool and having an inside diameter sized to receive the spool heads 32 with a tight iit, and a pair of circular end rings 38 secured by bolts 39 to the ends of the case body and formed with center holes 40 larger than the diameter of any part that is to pass through the die assembly. For optimum sealing, the spool may be made somewhat oversize and compressed within the case by the clamping action of the bolts.

As shown in FIGS. l-5, a supply line 41 from the pressure source opens into the case and into the groove chamber 33 therein through a fitting 42 screwed into an inlet port 43 in the body 37 in axial alignment with the groove 33. The backing pressure produced in the chamber is controlled during tapering from the smaller end toward the larger end by means of a pressure relief valve 44 (FIG. l) for draining uid from the chamber as the outward buckling and expansion of the yieldable annulus 31 tends to increase the pressure. Thus, the pressure is maintained substantially constant, automatically as an incident to the changes in the volume of the chamber.

With the foregoing arrangement, a tube-drawing operation may be started by placing a conventional mandrel 14, as shown most clearly in FIG. 6, inside a pointed tube 13 of the type shown in FIG. 7, the sizes of the mandrel and tube being correlated so that the larger end of the mandrel ts closely in the unpointed end of the tube with a stem 45 on the smaller end of the mandrel disposed within the pointed portion of the tube. Then the pointed portion is inserted through the die ring 17, as shown in FIGS. 1 and 2, and is gripped in the chuck 19, ready to be drawn, with the smaller end portion of the mandrel within the land 29.

Until this time, the pressure within the chamber 33 should be relieved for free insertion of the tube 13 in the relaxed die ring 17. Then the die assembly is pumped up to contract the land 29 against the tube with sufficient pressure to collapse the tube firmly against the mandrel 14. It should be noted that it is possible to apply pressure sufficient to clamp the tube against the mandrel so tightly that the drawing force on the tube will exceed the tensile strength of the tube. If this happens, a tube can be torn apart. Accordingly, the pressure applied should be limited to that which is sucient to collapse the tube and to continue such collapsing as the tube is drawn.

The appropriate pressure, of course, will lvary with the thickness and composition of the tubing. For example, for a relatively thin-walled aluminum tube, such as `6061-T6, 0.060 of an inch thick, being tapered from one inch to one-half inch, a pressure on the order of 5,000 p.s.i. in the 'die assembly has been sufficient, while a thicker-walled tube composed of steel may require pressure on the order of 20,000-2J5,000 p.s.i., or higher. Experience with a particular type of tubing quickly indicates the suitable pressure range.

After the die assembly 10 has been pumped up, the draw cylinder 23 is actuated to pull the tube 13 and the mandrel 14 as a unit through the die ring 17. Since the pressure is sufficient to collapse the tube, all of the tube wall is cammed and guided into the bell 30 of the die ring with a smooth, rolling action as shown at 47 in FIG. 4, and is ironed out onto the mandrel by the ring. As the diameter of the tapered portion of the tube grows, the minimum diameter of the die passage must grow correspondingly, and the original land area buckles outwardly away from the tube, as shown at 48 in FIG. 4, while the effective land progresses along the bell toward the entry end of the passage. Consequently, the actual area or band of tight, pressing engagement between the die ring and the tube remains relatively narrow.

This action continues as the die ring 17 moves relative to the tube 13 and the larger ends of the tube and the mandrel 14 approach the die ring, the conditions of the parts near the end of the drawing stroke being shown in FIG. 5 wherein it will be seen that the land has progressed to a point along the curvature of the bell where the diameter of the passage is approximately the same asA the diameter of the larger end of the tube. During further movement of the tube from this position, the die ring smooths the free end of the tube onto the mandrel and then slides olf the latter. -It should be noted that the die ring will remain substantially in the expanded condition until the pressure in the chamber is changed to return the ring to its original condition.

When the drawing operation has been completed, the pointed end portion 15 of the tube 13 is trimmed off to form a completed tube 13a, shown in lFIG. `8, having the desired taper according to the taper of the mandrel used. Circumferential distribution of any minor defects that may have been present in the original tube blank has been avoided, and the tapered wall has the same thickness as the wall of the original blank. In the production drawing of a series of identical tubes, the die ring now is returned to its original condition by changing the pressure in the chamber, and the die assembly 10l is ready for another draw.

Shown in FIGS. 9-11 is a die assembly that is identical to that shown in FIGS. l-5, the basic parts being indicated -with the same reference numbers. In this instance, however, the drawing operation is started adjacent the larger end of a mandrel 14 and progresses toward the smaller end. The only significant differences in this method of operation are the anchoring of the tube 13 to the larger end of the mandrel, and the use of a reVersely-tapered guide section 49 fastened to the larger end as a coaxial extension to which the stem 50 is connected.

As shown in FIGS. 9 and l0, the guide extension 49 has a coaxial counterbore 51 forming a seat in its larger end for receiving the larger tend of the mandrel with a clearance rit. A threaded stud 52 holds the two securely together, and the end of the tube 13 is clamped snugly in the clearance between the mandrel and the extension.

Starting with the die ring 17 in its contracted form, it is expanded to slightly larger than the size of the tube 13 on the mandrel 14, simply and quickly, by drawing the extension 49 through the ring until the land of the ring passes the right-hand end of the extension. This preliminary operation causes the yieldable annulus 31 to buckle outwardly, just as before, as shown at 31a in FIG. 10i, into the proper condition for engagement with the tubing. Then, after application of sufcient pressure to collapse the tube around the mandrel, the drawing operation proceeds as before. Of course, the yieldable annulus now is contracted progressively around the tube and the mandrel, by the backup pressure in the groove chamber 33t, and the land area travels progressively away from the entry end of the die passage as the diameter of the work decreases, returning substantially to the normal, contracted condition as the smaller end of the mandrel is drawn through the ring, as shown in FlIG. 11. To maintain the backing pressure substantially constant, a conventional accumulator 53 (FIG. 9) may be used to supply additional uid to the chamber at the selected pressure.

The primary advantage of this method of operation is the elimination of the need for a tube-pointing operation preparatory to drawing. By using the guide extension 49 to expand the ring 17 prior to contraction, the deformation of the ring is kept Within the same range of movement that is experienced in the method described in connection with lFIGS. l-5.

When the die assembly 10 is to be used in relatively heavy drawing operations, the die ring 17 will, of course, be designed for higher backing pressures and with correspondingly thicker wall sections in the yieldable annulus 31. More rigid plastic materials also may be used-for example, with ber reinforcement of the type used in the material sold as NylaiL In addition to sustaining the higher backing pressures, the die ring must withstand the greater axial forces tending to pull the land area through the die with the tube.

For greater axial strength under such loads, the die assembly 10 also may be modified, in the manner shown in FIGS. 12-14, with a plurality of radially movable, axially rigid braces 54 distributed around the die ring 17. Herein, these braces are sheet metal ngers shown most clearly in FIG. 14 as being of U-shaped transverse cross-section and being pivoted at their outer ends on pins 55 spanning the sidewalls of a groove 57 in the body 37a of the case. The fingers preferably are urged counterclockwise (FIG. 12) about the pivot pins 55 by suitable spring means (not shown) which hold the free ends yieldably under light spring pressure against the bottom wall 34 of the groove 33 around the die ring and is closefitting relation with the adjacent walls of the heads 32 on the spool.

Thus, the radially free ends of the lingers 54 are positioned close to the yieldable annulus 31 to resist axial deformation of the land, throughout the operation, while being movable radially so as to avoid interference with proper radial yielding of the die ring 17. In all other important respects, the construction and operation of the modified die assembly are the same as those previously described.

It has been stated that hard nylon is a suitable material for use in the die ring, and that reinforced nylon such as that sold as Nylafil has even higher ultimate yield strength for resisting failure under the forces to which the ring is subjected, these forces being the fiuid pressure introduced into the die assembly, the overall axial force tending to pull the ring out of the case and, most importantly, the axial extruding force to which the land area is subjected as a tube is drawn through the ring. Thus, the key characteristics of the die material are the ability to expand and contract repeatedly through the range necessary for a particular drawingy operation, and sufficient hardness and strength or toughness to transmit the backing pressure to the tubing without failing axially as the tubing is drawn.

Along with these characteristics, the die material should have a low coefficient of friction to minimize the drag and heat accompanying the drawing operation. With a nylon die, simple bar soap has been used as a satisfactory lubricant, whereas many conventional drawing operations are believed to require complex and sophisticated lubricant mixtures.,

Specifically, the material sold as Bunting Cadco is a preferred hard nylon for the die ring. It is believed that Delrin also will be satisfactory, and that there are various other plastic materials with the proper physical characteristics for the present invention. In view of the teachings herein, particular materials will suggest themselves to those skilled in the plastics art.

From the foregoing, it will be evident that the present invention provides both an improved die assembly and a novel tube-drawing method using an expansible and contractible die ring 17, backed by uniformly distributed pressure, to collapse a tube 13 around a mandrel 14 into a desired form as the tube and mandrel are moved endwise through the die ring, the backing pressure `being maintained substantially constant throughout the draw. In addition, the die ring is reusable many times for economical and efficient mass-production of tapered tubing, in contrast to prior sacrificial dies or other expansible metal dies that have limited reusabiilty at best. The result is a highly versatile die assembly that is believed to be a significant advance in the art.

It also will be apparent that, while a particular form of the invention has been illustrated and described, various modifications may be made without departing from the spirit and scope of the invention. Moreover, while the die assembly 10 is intended primarily for the tapering 0f tubing on mandrels, its usefulness is not necessarily so restricted.

I claim as my invention:

1. A die assembly for drawing a ductile metal tube of substantially constant diameter on a tapered mandrel having a predetermined diameter change to taper the tube according to the taper of the mandrel, said die assembly comprising:

an annular drawing spool composed of relatively hard,

non-metallic plastic material capable of stretching radially and being contracted back to original annular form repeatedly and Iwithin the range of said diameter change without rupturing or hardening, said spool having a central through-passage of circular cross-section including a central land, a curved entrance `bell flaring from said land toward one end of the passage, and a relief diverging from said land t0- ward the other end of the passage,

a circular head at each end of said spool,

and a peripheral groove circumferentially around spool separating said heads and having a curved bottom surface radially separated from said land by a wall of preselected thickness at the land and progressively increasing in thickness toward each end of the spool;

a rigid case encircling said spool and having walls abutting against said heads to support the spool rigidly within the case, the latter cooperating with said groove to define a sealed pressure chamber around said bottom surface;

said case having at least one port for admitting pressure fluid into said chamber;

means for delivering Huid through said port to said chamber under sufficient pressure to contract said land around a tube on a mandrel in said passage and collapse the tube around the mandrel;

and means for maintaining the pressure within said chamber substantially constant during relative movement of the tube and mandrel through said spool whereby the tube is compressed smoothly along its length and drawn to the taper of said mandrel.

2. A die assembly as defined in claim 1 in which said plastic material is hard nylon.

3. A die assembly as defined in claim 1 further including means on said case directly bracing said bottom surface against axial yielding while permitting free radial yielding thereof.

4. A die assembly as defined in claim 3 in which said bracing means comprise a plurality of fingers pivotally supported on said case around said groove and having radially swingable free end portions pressing yieldably against said bottom surface.

5. A die assembly for drawing a ductile metal tube of substantially constant diameter onto a tapered mandrel and into a tapered tube conforming to the taper of the mandrel, said die assembly comprising:

an annular die ring composed of relatively hard nonmetallic plastic material capable of stretching and being contracted .back to original form repeatedly and within a substantial range of movement without rupturing or hardening, said ring having a central through-passage of circular cross-section including a bell at one end converging to an internal land at the area of minimum diameter of said passage, and

said die ring having a peripheral groove circumferentially around said ring and forming a radially yieldable annulus encircling said land and having a peripheral outside surface;

a case encircling said ring and rigidly bracing the latter against axial yielding, and also cooperating with said groove to define a sealed pressure chamber around said annulus;

and means for producing a preselected fluid pressure Within said chamber sufficient to press said land against a tube within said passage and compress the tube around a mandrel Within the tube, and also for maintaining said press-ure substantially constant during drawing movement of the `tube and mandrel relative to said passage;

said plastic material having suliicient mechanical strength to resist substantial axial deformation of said land during such movement whereby the tube is ironed smoothly out onto said mandrel into the tapered form of the latter while said land yields under said preselected pressure to conform to the changing diameter of the tube.

6. A die assembly as defined in claim S in which said annulus is of minimum thickness at said land and increases progressively in thickness to-ward said one end whereby the increasing diameter of said mandrel causes said annulus to buckle outwardly and the land area of said passage to travel along said inside surface toward said one end.

7. A die assembly for use in drawing a tube, said die assembly comprising:

an annular die ring composed of relatively hard nonmetallic plastic material capable of being stretched and contracted in annular form repeatedly within a substantial range of movement without rupturing or hardening, said ring having a central through-passage including a bell at one end converging to a land,

a peripheral groove circumferentially around said ring,

and a yieldable annulus surrounding said passage and having said land on its inside surface;

a case encircling said ring and supporting the latter against axial movement, said case also having a sealed pressure chamber around said surface and said groove;

and means for applying a preselected uid pressure to said chamber and thus to said groove to back said land as a tube is drawn through said passage, whereby said land is capable of reducing the size of the tube in a drawing operation.

8. A die assembly as defined in claim 7 in which said land is centrally located within said annulus, and said inside surface flares arcuately in both directions therefrom, said annulus having its minimum thickness at said and progressively increasing in thickness along said bell.

9. A die assembly for drawing a ductile metal tube, comprising:

an annular die spool composed of non-metallic plastic having properties of hardness, repeated deformability and low coeicient of friction similar to nylon so as to be capable of being stretched radially and contracted back to original annular form repeatedly and within a substantial range, said spool having a through-passage of circular cross-section including a central land and a curved entrance bell flaring from said land toward one end of said through-passage,

a head at each end of said spool,

and a peripheral groove around the outside of said spool separating said heads and reducing the thickness of the spool adjacent said land;

means supporting said spool for longitudinal drawing movement relative to the tube and the mandrel;

and means for applying uid pressure in said groove su'iciently high to contract said die spool around the tube on the mandrel and collapse the tube against the mandrel, and for controlling said pressure as the die spool is moved along the tube.

10. A die assembly for drawing a ductile metal tube,

comprising:

an annular die spool having a through-passage including a central land and a curved entrance bell aring from said land toward one end of said through passage,

a head at each end of said spool,

and a peripheral groove around the outside of said spool separating said heads and reducing the thickness of the spool adjacent said land;

means supporting said spool for longitudinal drawing movement relative to the tube and the mandrel;

and means for applying backing pressure to said groove sufficiently high to contract said die spool around the tube on the mandrel and collapse the tube against the mandrel, and for controlling said pressure as the die spool is moved along the tube;

said spool being composed of relatively hard nonmetallic plastic material capable of stretching and being contracted back to original form repeatedly and within a. substantial range of movement without rupturing or hardening, and having suiciently low coeicient of friction and high mechanical strength to resist axial deformation during drawing of a tube.

11. A die assembly as defined in claim 10 in which said spool is composed of nylon.

12. A die assembly as defined in claim 10 in which said groove has a bottom wall that is curved in axial crosssection, the curvatures of said bottom wall and of said entrance bell cooperating to reduce the thickness of said ring progressively toward said land to a minimum thickness adjacent said land.

13. A die assembly as dened in claim 10 in which said supporting means for said spool include a case engaging said heads to brace the spool axially during said drawing movement.

14. A die assembly as dened in claim 13 in which said case also seals against said heads to form a pressure chamber defined in part by said groove and in part by said case.

15. A die assembly as defined in claim 10 in which the surface of said through-passage is smoothly curved from said entrance bell to said land and similarly but reversely curved beyond said land to form a relief, said groove having a curved bottom surface generally centered on the curvature of said bell and said relief, and of narrower width, whereby the thickness of said spool increases progressively in both directions from said land. 16. The method of forming a tube on a tapered mandrel to taper the tube according to the taper of the mandrel, said method comprising the steps of positioning a tube in telescoped relation with a tapered mandrel with the tube and the smaller end of the mandrel within a flexible die ring composed of relatively hard non-metallic plastic material capable of expanding and contracting repeatedly in annular form and having a peripheral groove; confining fluid in a chamber around the groove under sufficient pressure exerted on the ring through the groove to contract the ring around the tube and collapse the tub-e rmly against the mandrel;

moving the tube and mandrel endwise relative to the ring to pass the tube and mandrel through the ring from the smaller end toward the larger end of the mandrel;

and, during such movement, maintaining the pressure substantially constant by relieving uid from the chamber as the ring expands as the tube and mandrel pass through the ring, and thereby maintaining a substantially constant working force between the ring and the tube while collapsing the remainder of the tube around the mandrel.

17. The method of forming a tube on a tapered mandrel to taper the tube according to the taper of the mandrel, said method comprising the steps of:

positioning a tube in a telescoped relation with a tapered mandrel with the tube and the larger end portion of the mandrel within a llexible die ring composed of relatively hard non-metallic plastic capable of expanding and contracting repeatedly in annular form and having a peripheral groove; conning fluid in a chamber around the groove under suflicient pressure exerted on the ring through the groove to contract the ring around the tube and collapse the 'tube firmly against the mandrel;

moving the tube and the mandrel endwise relative to the ring to pass the tube and the mandrel through the ring from the larger end toward the smaller end of the mandrel;

and, during such movement, maintaining the pressure substantially constant by adding fluid under pressure to said chamber as the ring contracts progressively around the tube and the mandrel, thereby maintaining a substantially constant working force between the ring and the tube while collapsing the remainder of the tube around the mandrel.

References Cited UNITED STATES PATENTS Cimochoski 72-59X Buckingham 72-276 Hinshaw 7'2--274X Smith 72-468X 10 Strock 72-274 Green 72--468X 12 2,927,372 3/1960 Powell 72-84 2,597,623 5/ 1952 Dies 72--57/ FOREIGN PATENTS 17,090 7/ 1893 Great Britain 'l2-274% CHARLES W. LANHAM, Primary Examiner M. J. KEENAN, Assistant Examiner U.S. Cl. X.R. 72-370, 468 

